Waves: Mechanical, Pressure and Light
“In space no one can hear you scream”....Alien
Is there energy here?
Force?
Work?
Work = Energy?
Why not one big wave?
Matter is sticky!
How well it sticks
depends on the type of
material.......
Solids > Liquids > Gases
Ripples are partly due to
water sticking together.
As the first ripple rises,
some of the force is
resisted by the elastic
connections between
water molecules and the
ripple is “pulled down”
creating a trough.
slinky demo
Slinky
„
Potential energy converted to kinetic energy due to its
height.
„
Energy in elastic bonds cause the up and down vibration.
„
Basically, we get a conversion and reconversion of
energy back and forth between kinetic and potential
energy!
„
What happens to the air as the slinky passes through it?
Slinky cont.
„
As slinky passes through air it encounters individual
molecules.....what happens to these molecules as they
get whacked?
„
Some kinetic energy from slinky is converted into kinetic
energy in the molecules.
„
The end product is that the slinky, being quite heavier
than the molecules, will end up pushing the air molecules
in the same direction and about the same speed.
„
Air molecules will continue on in a straight line and at a
constant velocity even after slinky is gone until other
forces act on them. (sounds like laws of motion to me!)
Pressure waves
„
What ends up happening is a region of dense air being
pushed through the air.
„
But, no actual movement of molecules along the wave
occurs!!!! The energy is transferred from molecule to
molecule, kind of like dominoes falling.
„
Ultimately, as slinky bounces up and down and is losing
energy it causes regions of dense and less dense air
moving away from slinky.
„
The rythmic bouncing of slinky creates oscillations along
the slinky.....mechanical waves.
Wavelength, Frequency, Amplitude and Speed.
Wavelength is measured as distance.
Frequency of a wave is the number of
wavelengths that pass a set point per
second.
Amplitude, or strength of wave, is
measured as the difference between
ambient and the peak of the wave.
The higher the amplitude the more
energy is carried.
Speed = wavelength * frequency
units are called Hertz or kilohertz
1 Hertz = 1 wavelength per second
Summary of Waves
„
Carry energy by transferring the energy from
one molecule to the next.
{ eg. billiards
„
They are caused by vibrations in the energetic
source
„
They tend to have a cyclical pattern, with
measurable wavelength, frequency, amplitude
and speed.
Now for biology
„
Waves carry information and an organism may just glean
some information from this energy which may help them
survive and/or reproduce.
„
Several of our sense are designed just for this.
„
For example, the pressure waves we described.....any
other name for them? SOUND!
„
Our ears are designed to detect pressure waves ranging
from 20 – 20,000 Hz.
„
Type of energy
transfer?
Semicircular
canals
External
Auditory Canal
Cochlea
Tympanic membrane
Auditory Ossicles
(malleus, incus and
stapes)
“In space, no one can hear you
scream”....Alien
Is this true or false? Given what we have discussed what would
you say?
Is there air in space? Therefore there is nothing to transmit the
energy from one place to another. No waves = No sound
So far both mechanical and pressure waves depend on a fairly dense medium
to carry energy from one place to another. Here are three ways energy can
be transferred.
Conduction: The transfer of heat (kinetic energy) by physical
contact between molecules. Includes mechanical waves and
pressure waves.
Convection: The transfer of kinetic energy by moving groups of
molecules from one area to another.
Radiation: The transfer of energy via electromagnetic waves.
Waves - Electromagnetic
Electromagnetic waves transfer energy with no
physical contact.
Electromagnetic waves are a combination of a
electric and a magnetic fields.
Radio waves, visible light, X-rays, and all the other parts
of the electromagnetic spectrum are fundamentally the
same thing, electromagnetic radiation.
Relative sizes of wavelengths to things you know
Properties of waves
„
Reflection
„
„
Absorption
Refraction
Photoreceptors - eyes
Retina
Cornea
pupil
Iris
Optic
nerve
Rods: Respond unselectively to many wavelengths of light. Primarily detect
amplitude.
Cones: Respond to a narrow range of wavelengths; different types detect
different frequencies; responsible for color vision.
Cones require fairly high light levels to function well – thus in dim light, we see
primarily in Black-and-white.
You should be able to:
Identify wavelength, period, frequency, amplitude, and speed of a wave.
Explain how a vibrating object creates sound waves.
Describe how a mechanical wave carries energy from one place to another.
Trace the path of sound energy through the ear.
Identify key structures in the ear and how they affect sound waves or are affected
by them.
Identify/explain properties of three modes of energy transfer: Conduction,
Convection and Radiation.
Identify and explain the properties of electromagnetic waves: reflection, absorption
and refraction.
Trace the path of light through the eye.
Identify important structures in the eye and explain how light affects them or they
are affected by light.